Sensor is a device that transforms a measured quantity into a readable format, typically into an electrical signal. Nowadays, there are commercially available sensors virtually for any measurement purpose. According to the connectivity, sensors can be divided into wireless and wired sensors. Wired sensors are connected via wiring harnesses or cable assemblies to a reader device. Wireless sensors can be read without a physical connection to the sensor, and are often realized equipping the sensor with a radio transceiver. The transmitted radio signal is interpreted by a receiver which converts the wireless signal into a desired output. Wireless operation can be beneficial in many applications, where wired connection is difficult for example due to harsh operating conditions (like temperature and pressure, rotating parts, or cost and complexity of wiring. However, wireless sensors also have some drawbacks such as limited lifetime due to battery, limited read-out distance due to attenuation and interference, security issues because of the uncontrollable propagation of the signal and potentially low speed of communication. Based on the power source and communication principle, wireless sensors can be divided into three categories: active sensors, semi-passive sensors and passive sensors.
Active wireless sensors usually have both a radio transceiver and an on-board battery that is used to power up the transceiver. Active wireless sensors, having their own power sources, can use powerful transmitters and sensitive receivers. However, the battery on board limits the life time and also increases the size and weight. Due to more complex circuit, the price of an active sensor can be much higher than that of a passive sensor.
Semi-passive wireless sensors do not contain a radio transceiver, but are equipped with a battery. The battery is used to power up an integrated circuitry (IC) and enables the sensors to operate independently of the reader device or to maintain memory in the sensor. Semi-passive battery-assisted sensors utilize modulated backscattering technique for communication. This means that semi-passive sensors do not require any power from the on-board battery for transmission, but the sensor simply reflects back some of the power emitted by the reader device.
Unlike the active and semi-passive sensors, passive sensors do not require an on-board battery. Therefore they can be less complex, smaller, more inexpensive, and their lifetime is not limited by the power supply. The typical read-out distance of passive wireless sensors is between 10 cm and 3 m. Passive wireless sensors can be divided into four main categories: radio frequency identification (RFID) tags, electrical resonance circuit sensors, surface acoustic wave (SAW), harmonic sensors and intermodulation sensors.
SAW sensors utilize an interdigital transducer patterned on a piezoelectric substrate to convert the electromagnetic energy into a surface-acoustic wave (SAW). The SAW is then manipulated with acoustic reflectors, transformed back to electromagnetic energy, and radiated back to the reader device. The measured quantity affects the propagation properties of the SAW on the piezoelectric substrate. The need to use a piezoelectric material for the sensing element limits possible applications. In addition, SAW tags only enable hard-coded identification (ID), and they do not provide any memory to the chip.
Resonance sensors consist of a simple resonance circuit, whose resonance is sensitive to a measured quantity. These sensors require near-field coupling to the reader, which limits their read-out distance to a few centimeters.
Harmonic sensors, when illuminated by a reader with one or many tones, scatter back the sensor data at a harmonic frequency. In other words, the sensor mixes the tones (nonlinearity is needed in order to set mixing) and reflects a signal containing harmonic products, which are offset from the reader frequencies. The harmonic reader receives the harmonic of the transmitted frequencies, and the received data can then be processed to find the exact location and mobility of the object causing the generation of this harmonic. The concept was first proposed for telemetry. Recently, an intermodulation communication principle for sensing applications has been developed. In this principle, the sensor is actuated by two closely located frequencies, and the sensor data is scattered back at an intermodulation frequency. This approach enables a wireless read-out of a generic sensor element (such as a MEMS sensor) across a very large distance (even tens of meters). The sensors can also be equipped with an ID. However, the concept requires a special reader, and it does not provide memory or anti-collision protocol. Such approach is disclosed in WO2011/121180 and U.S. Pat. No. 6,378,360.
RFID is an identification technology that uses radio waves to communicate between tags and a reader and it is used to identify items. There are a few advantages of RFID over optical barcode identification such as no line-of-sight is required between the reader device and the tag, and the RFID reader can also read hundreds of tags at a time. Passive RFID tags utilize the modulated backscattering communication principle which is illustrated in FIG. 1. When a tag 10 communicates with an RFID reader 11, it modulates the received signal 12 and reflects a portion of it 13 back to the reader. A typical passive tag consists of an antenna connected to an application specific microchip. When wirelessly interrogated by an RFID transceiver, or reader, the RFID tag antenna receives power and RF signals from the RFID reader and provides them to the chip. The chip processes the signals and sends the requested data back to the RFID reader. The backscattered signal is modulated according to the transmitted data. The highest operation frequency and read-out distance of RFID are limited by the rectified power for the integrated circuit (IC) and are a few GHz and 5-10 m, respectively.
RFID is mostly used for identification. RFID tags are equipped with a rewritable memory, which enables the reusability features of RFID tags, but they are not useful for measuring external quantities. RFID has also been shown to be suitable for sensing by equipping an RFID tag with an external sensor and digital logic to read the external sensor. The advantage of this approach is that it would use a generic sensor element and thus would be well suited for a very broad range of applications. In this approach, however, an additional A/D converter and digital circuitry has to be included to the tag in order to enable sensor read-out. Increased power consumption due to the additional electronics reduces the read-out range significantly (e.g., from 10 m to 0.3 m with an 8-bit A/D converter). An additional sensor element further increases power consumption. Implementation considerations of the A/D converter and additional digital circuits are discussed in [1]: Chapter 9 “Smart RFID Tags”, in the book “Development and Implementation of RFID Technology”, ISBN 978-3-902613-54-7, February 2009, I-Tech, Vienna, Austria. http://www.intechopen.com/books/development and implementation of rfid technology.
To summarize, the current passive wireless sensor techniques have several limitations. No single technique can simultaneously provide sensing, the sophisticated features of the RFID technology, such as identification and anti-collision, and large read-out distance.